A key witness in a war crimes trial in Bangladesh, who disappeared late last year, is being held in an Indian prison after being tortured, according to Human Rights Watch.

Shukhoranjan Bali claims he was abducted by Bangladeshi police outside the International Crimes Tribunal building in Dhaka, Bangladesh, in November and later handed over to India’s Border Security Force who detained and tortured him, the New York-based rights group said.

According to local media reports, the government and the tribunal authorities have denied that Mr. Bali was abducted from outside the ICT.

Mr. Bali is currently being held in Dum Dum jail in Kolkata, where he is imprisoned for entering the country illegally, according to Meenakshi Ganguly, South Asia director for HRW.

According to Ms. Ganguly, the Bangladeshi national has served out his 110 day sentence for illegally entering the country, but he is still being held in jail. It is unclear why he remains in prison.

In a statement on its website, HRW expressed concern that Mr. Bali could be deported to Bangladesh at great peril to his life.

The rights group called on the Indian government Thursday to offer Mr. Bali protection until it is clear that his life will not be at risk if he returns to Bangladesh.

Mr. Bali was a key prosecution witness in the trial of Delwar Hossain Sayeedi, an Islamist politician and one of the country’s best known preachers, who has since been sentenced to death by the ICT, a specially convened court to investigate suspected war crimes committed during the country’s 1971 war of independence. But after changing his statement he was called by the defense to give evidence, Ms. Ganguly said.

It is unclear why Mr. Bali changed his statement midway through the trial.

“We feel that he is at risk. For India, the easiest thing is for them to send him back [to Bangladesh]; but if he is sent back, then both sides of this trial are very interested in what he has to say,” Ms. Ganguly added.

Mr. Bali told independent lawyers working with Bangladeshi nationals in Dum Dum that he was kidnapped before he was about to give evidence in the trial of Mr. Sayeedi, who was accused of killing his brother, Ms. Ganguly said.

Neither the Border Security Force nor the Home Ministry responded to requests for comment.

According to a report by HRW published in 2010, more than 900 Indians and Bangladeshis were killed in the previous decade by India’s Border Security Force.

“Hundreds of complaints of mistreatment by the border forces have been filed, but no member of the force has been prosecuted,” a statement released by HRW to accompany the report said.

Dum Dum jail could not immediately be reached for comment. The Bangladeshi police force could not immediately be reached for a response to the allegations.

Mr. Sayeedi was found guilty in March of eight counts including murder, arson, rape and religious persecution during the 1971 war, lawyers connected to the tribunal said.

Hundreds of thousands of civilians died during the war, many of them at the hands of Islamist militia who opposed independence and wanted Bangladesh to remain part of Pakistan.

Prime Minister Sheikh Hasina’s government set up the war crimes tribunal two years ago and pledged it would adhere to standards of international justice.

It was not clear whether Mr. Bali’s return would allow Mr. Sayeedi to appeal his death sentence, which caused widespread protests in the country.

HRW said in a statement Thursday that India should not return Mr. Bali to Bangladesh until he is interviewed by the Indian office of the United Nations High Commissioner for Refugees (UNHCR). The rights group wants to give Mr. Bali the opportunity to claim asylum or refugee status. If he does not wish to claim asylum, or his asylum claim is rejected, India should still not return him to Bangladesh, the statement said.

“Those involved in his abduction may have assumed Bali would be killed by the Indian Border Security Force when he was pushed into India, or that he would permanently disappear,” Brad Adams, Asia director of HRW said in the statement.

“There is a real risk to Bali if he is returned to Bangladesh, as he could expose those involved in his abduction. Bali needs access to an independent lawyer and UNHCR so that he can make an informed decision about whether it is safe to return to Bangladesh,” Mr. Adams added.